1. Field of the Invention
The present invention relates to a secondary battery.
2. Description of the Related Art
In a secondary battery two secondary cells may be laminated and electrically connected to a protective circuit module to form a battery pack. When a plurality of secondary cells is laminated to be electrically connected to each other, the connection may be performed by spot welding.
Spot welding is a kind of resistance welding mainly used to weld metal members. The principle is that members to be welded are melted and welded by Joule heat generated when current flows through the members for a short time. The spot welding is classified into a single phase AC series method and a DC direct method. In the direct method, two members to be welded are brought into surface contact with each other and two welding rods contact rear sides of the members to face each other, and electric current is applied thereto to perform welding. In the series method, when two members to be welded are brought into surface contact with each other and two welding rods contact a side of one of the members in parallel, electric current is applied to perform welding. The direct spot welding is used when welding space is guaranteed at both sides of the members to be welded. The series spot welding is used when welding space is not provided at a side of the members to be welded.
In general, the series spot welding may be performed during the manufacturing of a secondary battery. Since, in the series spot welding, two welding rods may not be parallel to each other, workability is enhanced and welding may be easily performed even at a narrow space. However, in the series spot welding, when a welding length is increased due to a thick material to be welded, welding force decreases.
The above information disclosed in this Related Art section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.